1. Field of Invention
The field of the currently claimed embodiments of this invention relates to systems and methods for high-throughput production of microbeads.
2. Discussion of Related Art
Microencapsulation is an attractive way to deliver cells and biologics into the human body, providing immunoisolation while allowing biomolecules, either preloaded or synthesized in situ by cells, to be released in a controlled manner. Typical microencapsulation involves injecting alginate from a syringe into a bath of calcium solution. While the generation process is rapid, the size distribution is extremely polydisperse. In addition, gel beads smaller than 300 microns cannot be routinely made, making injection through small bore catheters almost impossible. There remains a need for systems and methods to generate uniform microbeads that are small (<100 μm) at quantities that are suitable for clinical applications.
Reported microbead generation on microfluidic chips often involve fusion of manually-synchronized alginate and calcium droplets in oil, which severely limits the rate of bead generation (Zhao L B, Pan L, Zhang K, Guo SS, Liu W, Wang Y, et al. Generation of Janus alginate hydrogel particles with magnetic anisotropy for cell encapsulation. Lab Chip 2009 Oct. 21;9(20):2981-2986). Generation frequencies range from single hertz to a theoretical maximum of 4 kHz for inkjet-based techniques, although in practice Dohnai et al. only achieved 30 Hz (Dohnal J, Stepánek F. Inkjet fabrication and characterization of calcium alginate microcapsules. Powder Technology;200(3):254-259). Centrifugation-based generation results in comparable frequency to our method at 600 Hz, but can only prepare beads with diameters of hundreds of microns (Haeberle S, Naegele L, Burger R, von Stetten F, Zengerle R, Ducree J. Alginate bead fabrication and encapsulation of living cells under centrifugally induced artificial gravity conditions. J Microencapsul 2008 Jun.;25(4):267-274). On the other hand, a recent report used oleic acid to act as both continuous phase (with alginate as the discrete phase) and calcium reservoir (Kim C, Lee K S, Kim Y E, Lee K J, Lee S H, Kim TS, et al. Rapid exchange of oil-phase in microencapsulation chip to enhance cell viability. Lab Chip 2009 May 7;9(9):1294-1297). Although it solved the problem of synchronizing the different aqueous inlets, clogging at the droplet generation nozzle was a significant problem, especially at higher generation frequencies, where a delicate balance of flow pressures must be maintained.
Another challenge in high frequency generation is bead aggregation, since only partial crosslinking occurs at high generation frequency due to relatively low saturation concentration of calcium ions in the oil phase. There thus remains a need for improved systems and methods for producing microbeads.